cache hex packages in travis

.travis.yml

@@ -10,6 +10,9 @@ script: "./rebar3 ct"
 branches:
   only:
     - master
+cache:
+  directories:
+  - $HOME/.cache/rebar3/hex/com/amazonaws/s3/s3.hex.pm/tarballs/packages/
 before_deploy: "rm -rf !(rebar3)"
 deploy:
   on: